Habit
A behaviour your brain runs on autopilot when it sees a cue. Built by repetition, not by wanting.
Do something in the same situation enough times and the brain hands it to a cheaper system: cue, action, done, no decision involved. Coffee when you sit down, phone when you're bored, snapping when you're tired. This is how the brain saves effort, and it saves it on good and bad habits alike. Evidence: solid
The "21 days" figure comes from a 1960s surgeon's anecdote, not a study. In the best-known study, new habits took anywhere from under three weeks to most of a year to become automatic, with a middle of around two months. Evidence: decent
Why it matters
You don't beat a habit with willpower, because willpower is the expensive system and the habit is the cheap one. You change the cue, add friction, or put a different action on the same cue. Evidence: decent